This invention relates to cardiac pacemakers and more particularly to improvements in dual chamber pacing apparatus.
Dual chamber pacemakers are used in those patients suffering from complete heart block and those who can benefit from an increase in ventricular rate and cardiac output in accordance with an increase in physical activity. The typical dual chamber pacemaker essentially comprises dual demand pacers, one for the atrium and one for the ventricle. Each demand pacer includes a sensing element and a pacing element positioned in its respective chamber. Each pacer has an intrinsic escape interval, that is a lower rate limit for the sensed atrial beats. If the time between suceeding atrial beats exceeds the refractory period of the pacer, the pacer will initiate a paced beat. For example, with the pacemaker escape interval set at 0.86 seconds (corresponding approximately to 70 beats per minute), the pacemaker itself will initiate a beat if it does not sense a spontaneous beat within this time period. Consequently, during occurrences of bradycardia or asystole, the pacemaker will output paced beats once every 0.86 seconds, thereby causing the paced chamber to beat at approximately 70 beats per minute.
For dual chamber pacemakers, the pacemaker atrial and ventricular output beats are synchronized. For example, in patients with normal sinus rhythm and heart block, the atrial demand pacemaker functions on a standby basis. That is, it senses the atrial beats but does not generate atrial pacing pulses because of the presence of the sinus beats. Meanwhile, the ventricular sensor awaits a ventricular beat. If the beat is not sensed within a predetermined, artifically induced A-V delay period (typically 200 milliseconds), the ventricular pacing electrode will generate a pacing pulse. As a result, the heart chambers beat in closely approximated natural synchronism.
It has been estimated that at least 45% of all patients who require a pacemaker implant have 1:1 retrograde V-A conduction, including approximately 67% of those with sick sinus syndrome and 14% of those with complete antegrade heart block. This means that in those patients with retrograde V-A conduction, the pulse which stimulates the ventricle is conducted back through the A-V node into the atrium. When this occurs in patients using the dual demand pacemakers, the atrial pacemaker senses this retrograde pulse as an atrial pacing pulse. This triggers the artificial A-V delay causing the ventricular pacing electrode to generate a pulse at the end of the preset 200 millisecond delay period. Because of the retrograde V-A conduction, this pulse will also be conducted back to the atrial sensing electrode causing the cycle to be repeated.
This phenomenon can likely result in pacemaker mediated tachycardia, causing the ventricle to contract every 400 milliseconds. That is, the ventricle pacemaker will output a pacing pulse every 200 milliseconds after an atrial pulse is sensed; and, due to the retrograde V-A conduction of the ventricular pulse, the atrial sensing will occur 200 milliseconds after each pacemaker generated ventricular pacing pulse. This 400 millisecond cycle causes the ventricle to contract approximately 150 times per minute. This phenomenon at best causes much discomfort and anxiety in the pacemaker patient; and, at worst, may cause severe pathological problems. Consequently, it can be seen that an improved dual demand pacemaker, which does not participate in the onset of tachycardia, is highly desirable.
Another anomalous phenomenon exhibited by the human heart is paroxysmal atrial tachycardia (PAT) which results from high rate pacing beats spontaneously generated from a location in the upper portion of the A-V node. PAT can be relieved by application of premature atrial pacing pulses or high rate atrial pacing pulses which are applied in bursts. Consequently, pacemakers are available which measure the atrial beat rate and apply atrial pacing pulses if the measured atrial rate exceeds a predetermined level. The problem with this type of pacemaker is that high atrial beat rates can be caused by other conditions such as ventricular tachycardia in patients having retrograde V-A conduction or under circumstances wherein the atrial pacing rate sensor is actually sensing the ventricular tachycardia waveforms due to a far field effect. In these situations, the application of a burst of high rate atrial pacing pulses would be ineffective. Also, high atrial heat rates can be the result of a normally elevated sinus rate due to physical exertion. In this case, atrial burst pacing is neither necessary nor desirable. Consequently, an improved pacemaker which can uniquely determine the onset of atrial tachycardia is also highly desirable.